The present invention is a method for compacting silica fume emitted from a smelting process into a form suitable for use as a reinforcing agent in concrete. The method involves the feeding of the silica fume through a horizontal feed means to a pair of vertically juxtaposed pressure rolls having surface depressions positioned so that the surface depressions of one roll correspond to the undepressed surface portions of the other roll.
In the production of materials having a high silicon content such as silicon, ferrosilicon, silicon carbide, and other silicon-containing alloys in smelting furnaces, there is generated a great deal of silicon monoxide which is converted to silicon dioxide. The silicon dioxide is in a very fine form and it is normally referred to as colloidal silica.
Because of the very light nature of colloidal silica, it does not remain in the smelting process but rather is carried up with the off gases from the smelting process into the furnace flue. Because escaping colloidal silica would be an environmental pollutant, it is necessary that the colloidal silica be recovered from the smoke from the smelting furnace. Typical dry methods employed in this regard involve bag house filters and similar means.
The very fine colloidal silica recovered, which has a typical weight by volume of 150 kg/m.sup.3 to 200 kg/m.sub.3, must then be disposed. The recovered colloidal silica is useful as a reinforcing agent for concrete. However, the use of colloidal silica in this application suffers from the disadvantage that due to the low density it is expensive to ship and store and is difficult to handle.
Accordingly, it is desirable to provide a method for increasing the density of the colloidal silica in order to reduce shipping and storage cost and to facilitate handling. Furthermore, it is desirable to provide a method to densify the colloidal silica such that the densified material is suitable for use as a reinforcer for concrete.
An apparatus for densifying and granulating powered materials is disclosed in Oldham et al., U.S. Pat. No. 3,114,930. Oldham et al. rely on a conical chamber having a rotary roll feed screw and maintained under vacuum to aerate and densify the powdered material prior to feeding of the powdered material to the juxtaposed pressure rolls.
Loffler. U.S. Pat. No. 3,632,247, describes a process where powders are compressed and deaerated between vacuum cylinders which are arranged in groups requiring different vacuum and connected to a common vacuum line. Valve control means in the vacuum line automatically and continuously adjust the vacuum pressure for the group of vacuum cylinders.
Carter, U.S. Pat. No. 3,664,385, compacts finely divided particulate material by utilizing a rotating screw feeder. The particulate material is advanced axially along a sleeve with the interstitial air between the particles in the sleeve at an internal sleeve pressure. Suction pressure relatively lower than the internal sleeve pressure is applied to the exterior of the sleeve to withdraw air from between the particles of the material to effect compaction.
Kongsgaarden, U.S. Pat. No. 4,126,423, discloses a method for compacting silica dust where the dust is charged to a drum having closed ends and is tumbled therein. Kongsgaarden. U.S. Pat. No. 4,126,424, describes a process for increasing the bulk density of silica dust where the silica dust is charged to a hopper and pressurized air is injected into the hopper at a force sufficient to fluidize the silica dust.
Leon et al., U.S. Pat. No. 4,325,686, disclose a powder densifying apparatus comprising a pair of opposed gas-permeable belts arranged to either side of a common axis so as to define a generally convergent densifying zone between their adjacent faces. The gas-permeable belts are driven toward the convergent end of the densifying zone at substantially equal speeds while powder material to be densified is fed into the divergent end of the densifying zone at a rate sufficient to maintain a substantially complete fill.
Kratel et al., U.S. Pat. No. 4,326,852, provide a method for increasing the bulk weight of silicon dioxide by means of sub-atmospheric pressure applied at a filter face, where the silicon dioxide is moved by means of a conveyer screw, whose longitudinal axis is arranged parallel with respect to the filter face and which preferably has a decreasing thread pitch in the feeding direction.
The principle advantage provided by the present method for compacting colloidal silica is its simplicity. The inventors have found that colloidal silica can be compacted in a form suitable for concrete reinforcement by merely feeding the colloidal silica by a horizontal feed means through a pair of vertically juxtaposed pressure rolls having surface depressions positioned so that the surface depressions of one roll correspond to the undepressed surface portions of the other roll. The described method does not require pre-compaction techniques using increased or decreased pressures or filtration as described in the prior art. Also, the present method can be conducted as a continuous process.